Radon is a gaseous element having the atomic number 86, i.e., an atom of radon has 86 protons in its nucleus and 86 electrons. Radon is a member of a group of elements known as the noble gases because they are relatively unreactive. Radon exists in the form of eighteen different isotopes. Isotopes are atoms of an element that contain different numbers of neutrons in their nuclei. Particular isotopes are commonly identified by their total number of protons and neutrons. For example, radon-222 is an isotope containing 86 protons and 136 neutrons.
All isotopes of radon are radioactive. Radioactivity is a process in which atoms undergo spontaneous nuclear transformations or decay by emitting atomic particles and/or electromagnetic energy. The most common types of radioactive decay are alpha decay (which produces an alpha particle consisting of two protons and two neutrons) and beta decay (which produces an electron). Electromagnetic radiation in the form of a gamma ray is also emitted as part of each alpha and beta decay. Each gamma ray is emitted in a random direction and travels in a straight line until absorbed.
The rate of radioactivity is measured by its half-life. A half-life is the time for one-half of the atoms to undergo radioactive transformation. Seventeen of the eighteen radon isotopes have half-lives of a minute or less. Radon-222 is the most stable of the radon isotopes and has a half-life of 3.8 days. Radon-222 decays through several intermediates (also known as decay progeny) into lead-210, an isotope of lead (atomic number 82) having a half life of 22 years. Lead-210 decays through intermediates into the stable, nonradioactive lead-206.
Radon is constantly being formed by the radioactive decay of subterranean uranium (atomic number 92). Uranium is present primarily in the form of the uranium-238 isotope. Uranium-238 is radioactive with a half life of 4.5 billion years. Uranium-238 decays through several intermediates into radon-222. The slow decay of uranium-238 has been occurring since the earth was formed. The decay chain from uranium-238 via radon-222 to lead-206 produces multiple alpha, beta, and gamma emissions. The highest energy gamma ray emission in this decay chain is about 7.7 MeV (mega electron volts).
Radon is also being formed by the radioactive decay of subterranean thorium (atomic number 90). Thorium is present primarily in the form of the thorium-232 isotope. Thorium-232 is radioactive with a half life of 14 billion years. Thorium-232 decays through several intermediates into radon-220. Radon-220 has a half-life of about one minute. Radon-220 decays through several intermediates into stable, nonradioactive lead-208. The decay chain from thorium-232 via radon-220 to lead-208 also produces multiple alpha, beta, and gamma emissions. For brevity, the term “radon” is used hereinafter to refer to all the isotopes of radon.
As a result of the ongoing formation of radon from radioactive decay, radon gas is constantly seeping upward through rock and soil toward the surface of the earth. The radon poses no risk if it decays before reaching the surface because its decay progeny are solids that remain wherever formed. Similarly, the radon poses no risk if it reaches the atmosphere because its concentration is so small. However, radon can enter buildings and concentrate to dangerous levels in the air, particularly in basements and first floors of buildings without basements. Radon levels vary considerably at different sites, and over time at any given site. Many factors cause these variations. For example, low pressure atmospheric conditions which often occur during storms are believed to draw higher levels of radon from the ground.
Radon is the leading environmental cause of cancer in the United States and the second-leading cause of lung cancer. The harmful effects are due primarily to alpha, beta, and gamma ray emissions inside the body resulting from radon breathed into the lungs. The health risks posed by radon have become more widely recognized in the past decade. The United States Environmental Protection Agency has recommended that homeowners take corrective action if the level of radon in their homes exceeds 4 picocuries per liter (about 0.04 decays per second per liter of air). There are two basic ways to lower radon levels in a building. The first is to suppress the flow of radon into a the building. The second is to remove the radon that is already there.
One common technique for suppressing the flow of radon into a building is to seal cracks and other openings in the building foundation, often in conjunction with sub-slab decompression. However, it is difficult to identify and permanently seal every opening. Furthermore, normal settling of buildings creates new openings and reopens old ones. The flow of radon into buildings is also suppressed by placing a barrier film on or under the lowest floor. An example of such a method is disclosed in Shahar, U.S. Pat. No. 6,676,780, Jan. 13, 2004.
The most common technique for removing existing radon from the air in a building is to increase ventilation. Simply opening doors and windows can lower radon levels. However, ventilation is difficult in basements with few, if any, windows or doors. Ventilation also results in the loss of conditioned air, discomfort, security problems, and increased costs of conditioning outside air.
Another technique for removing existing radon from the air in a building is to operate a liquid-gas contacting apparatus, commonly known as a wet scrubber. A wet scrubber is an apparatus in a which a gas stream is brought into contact with a working (scrubbing) liquid by forcing the gas through the liquid, by spraying the gas with the liquid, or by some other contact method. As the gas and liquid make contact, one or more components of the gas are absorbed, captured by the clathrate mechanism, or otherwise transferred from the gas to the liquid. The terms “absorb” and “dissolve” are used herein to refer to any mechanism by which a component of a gas becomes a component of a liquid. The removal of radon from air with a wet scrubber having an oil as the working liquid is disclosed in Gross et al., U.S. Pat. No. 5,743,944, Apr. 28, 1998. Gross et al. disclose that suitable oils include vegetable oils, animal oils, and petroleum oils.
While a wet scrubber removes radon from the air, the radon absorbed into the liquid remains a health risk. If the level of radon or its decay progeny reach a level to generate 2000 picocuries per gram (pCi/g), the liquid becomes a “low-level radioactive waste” as defined by the U.S. Environmental Protection Agency and requires special handling. Gross et al. address this problem by removing the working liquid from the wet scrubber, agitating and heating the liquid to release the radon from the liquid, and then venting the released radon to the atmosphere. The immediate removal of radon from the working liquid is an expensive step that adds greatly to the complexity and cost of the wet scrubber system.
Accordingly, a demand exists for an improved wet scrubber apparatus and method for removing radon from a gas, such as the air in the interior of a building, that does not require the immediate removal of radon from the working fluid.